The data plane of a forwarding element in a network defines the way that packets will be forwarded by the forwarding element through the network. In some networks, the data plane is defined at the forwarding elements based on control plane data received from network controllers. The network controllers define a control plane for the forwarding elements based on a desired network state and distribute the control plane to the forwarding elements in order for the forwarding elements to implement the network state in their respective data planes. The forwarding elements forward data messages (e.g., Ethernet frames, Internet Protocol (IP) packets, Transmission Control Protocol (TCP) segments, User Datagram Protocol (UDP) datagrams, etc.) through the network based on their respective data planes, as defined according to the current network state.
Network controllers (like any other computing devices) may occasionally fail. At this point, a new network controller will take over the provision of control plane data to the forwarding element(s). Ideally, this failover should result in a minimum of churn (e.g., data plane recalculation) for the forwarding elements.